The present invention relates to charge coupled devices (CCD) and in particular to the use of such devices for subtracting one signal from another.
One of the applications of charge coupled devices has been as registers for processing signals in an analog-to-digital or digital-to-analog converter. Typically in such a converter an analog signal is converted into its digital representation by successively comparing it with a reference signal. Heretofore, it has been necessary to perform the comparison by means of a differential amplifier external to the CCD devices.
In anapplication filed by Paul R. Prince on Sept. 15, 1976, Ser. No. 723,356, assigned to the assignee of the present invention, there is disclosed and claimed a circuit which is integrated as part of a CCD register which is capable of subtracting adjacent charges being propagated along that register.
It is an object of the present invention to sense the difference between the charges in two or more CCD registers directly, without using an external differential amplifier.
A related object of the present invention is to make possible an improved CCD analog-to-digital converter in which the function of subtracting a reference voltage from a signal voltage is performed by CCD registers without the use of an external differential amplifier.
A further object of the present invention is to compare the signals of two or more CCD registers by means of a circuit whose principal sensing element is incorporated in those registers.
These and other object of the invention are attained by a signal subtracting system comprising a semiconducting substrate in which there are formed at least first and second charge transfer channels. Respective sets of charge transfer electrodes extend over the charge transfer channels, with a normally floating sensing electrode structure extending over all of the channels. Clocking signals in the form of pulse trains are applied to the respective sets of charge transfer electrodes, with those channels whose signals are to be added being clocked out-of-phase with those channels whose signals are to be subtracted, so as to alternately shift charge packets from the first group of channels and the second group of channels to and from the substrate region which underlies the sensing electrode structure. A source of switched clamping voltage is connected to the sensing electrode structure and is timed to clamp it each time a charge packet is shifted from one of the groups of channels to the substrate region underlying the floating electrode structure and so as to allow the sensing electrode structure to float when a charge packet is shifted under it from the other group of channels. The difference between the charges in the first group of channels and the second group of channels is detected by detecting the voltage assumed by the sensing electrode structure when it is not clamped.
The present invention also resides in the method of subtracting a first signal from a second signal by generating first and second pulse trains corresponding to the amplitudes of those signals, providing in a semiconducting substrate at least first and second individual charge flow channels, extending an equipotential electrode structure over both of the channels, alternately clamping and floating the electrode, alternately stepping first and second trains of charge packets corresponding to the first and second pulse trains along the first and second charge flow paths respectively, into and out of the substrate region under the equipotential electrode structure, so that each time the electrode structure is clamped, charge is shifted under it from one of the channels and each time it floats a charge is shifted under it from the other of the channels, and detecting the potential of the electrode structure when it floats.